1. Field of the Invention
This invention relates to certain combinations of sparingly soluble calcium phosphates that are unique in their application as remineralizers of caries lesions in dental enamel and partially demineralized dentin and cementum and in their application as dental cements. When used as a remineralizer the present invention not only prevents tooth decay, but can also restore the lesions caused by dental caries. The dental cements of the present invention have a variety of dental applications, but are most useful where contact between the cement and living tissue is required.
2. Description of the Prior Art
When an incipient lesion or cavity develops on the surface of a tooth, the dentist traditionally fills the cavity that forms. This procedure may prevent the decay from spreading further, but does not restore the tooth to its original state. A considerable amount of research, however, has recently been directed to the remineralization of incipient dental lesions. The object of remineralization is to deposit Ca.sub.5 (PO.sub.4).sub.3 OH, known as hydroxyapatite, on the caries lesion such that the dental enamel incorporates the hydroxyapatite into its structure at the point of lesion. (Tooth and bone minerals are impure forms of hydroxyapatite). Thus, remineralization prevents further tooth decay and restores the tooth.
Remineralization of tooth enamel has been carried out experimentally both in vivo and in vitro. These studies have concentrated on the remineralizing properties of saliva and synthetic solutions supersaturated with respect to hydroxyapatite. Two recent articles that give a good overview of this research are Briner et al, "Significance of Enamel Remineralization", J. Dent. Res. 53:239-243 (1974); and "Silverstone, "Remineralization Phenomena", Caries Res. 11 (Supp. 1): 59-84 (1977). Additional experimental work in the areas of remineralization of calcium phosphate biomaterials may be found in Gelhard et al, "Rehardening of Artificial Enamel Lesions in vivo", Caries Res. 13: 80-83 (1979); Hiatt et al, "Root Preparation I. Obduration of Dentinal Tubules in Treatment of Root Hypersensitivity", J. Periodontal. 43: 373-380 (1972); LeGeros et al, "Apatitic Calcium Phosphates: Possible Dental Restorative Materials", IADR Abstract No. 1482 (1982); Pickel et al, "The Effect of a Chewing Gum Containing Dicalcium Phosphate on Salivary Calcium and Phosphate", Ala. J. Med. Sci. 2: 286- 287 (1965); Zimmerman et al, "The Effect of Remineralization Fluids on Carious Lesions in vitro", IADR Abstract No. 282 (1979); and U.S. Pat. Nos. 3,679,360 (Rubin) and 4,097,935 (Jarcho).
Generally, the supersaturated solutions or slurries used for remineralization experiments have been prepared from a single form of calcium phosphate. When a caries lesion is flooded with one of these supersaturated solutions, the calcium and phosphate ions in the form of precipitated hydroxyapatite remineralize the lesion. However, these solutions are impractical for use on patients for several reasons. First, the amount of calcium and phosphate ions available for remineralization in these supersaturated solutions is too low. It takes approximately 10,000 unit volumes of the usual supersaturated solution to produce one unit volume of mineral. Thus, remineralization by this method requires both an excessive volume of fluid and an excessive number of applications. The supersaturated solutions are inherently limited in this respect because they cannot maintain their supersaturated state. When the hydroxyapatite precipitates out to the point where the solution is no longer supersaturated, new supersaturated solution must be introduced or the remineralization process stops.
An example of another kind of problem is described in Levine, "Remineralization of Natural Carious Lesions of Enamel in vitro", Brit. Dent. J., 137: 132-134 (1974), where a phosphate buffer solution saturated with respect to CaHPO.sub.4.2H.sub.2 O (dicalcium phosphate dihydrate or brushite) and containing some fluoride was applied to dental enamel. To effect complete mineralization, exposure to the solution for three minutes every hour for 24 hours was necessary. Though the article suggested that this exposure could be achieved by use of two minute mouth rinses twice daily over the course of a year, this was admitted by the author to be an impractical procedure.
Another problem with single calcium phosphate slurries is that as the hydroxyapatite precipitates out of solution, the pH of the solution changes. Unless the old solution is removed from contact with the tooth material, the solution may become too acidic or alkaline and damage the dental tissue.
Another problem with known remineralization techniques is that the remineralization may stop before the lesion is completely remineralized due to build up of the remineralized tooth material in or on the outer layer of the tooth's surface. This build up occurs when the rate of remineralization is too fast and prevents the diffusion of the mineral into the deeper regions of the lesion, thus thwarting the full remineralization of the tooth.
There is a need for a method of remineralizing dental enamel that does not require excessive amounts of solution and inordinately long or frequent exposure times. Furthermore there is a need for a remineralization solution or slurry that can maintain a relatively constant pH and remain in a supersaturated state so that hydroxyapatite may be precipitated for a substantial period of time.
In the area of dental cements, the prior art shows an array of compounds. Some cements, however, irritate the pulp and are unsuitable for applications where the cement must come in contact with exposed pulp. Guide to Dental Materials and Devices, 7th Ed. (ADA 1974) p. 49. One solution to this problem is a cement made of materials similar in composition to tooth and bone mineral, since this would not irritate the living tissue.
The use of .beta.--Ca.sub.3 (PO.sub.4).sub.2 was suggested for pulp capping in Driskell et al, "Development of Ceramic and Ceramic Composite Devices for Maxillofacial Application", J. Biomed. Mat. Res. 6: 345-361 (1972); and the use of Ca.sub.4 (PO.sub.4).sub.2 O was suggested by the inventors in IADR Abstract. No. 120, J. Dent. Res. 54: 74 (1975) as a possible pulp capping agent. As described in the latter, Ca.sub.4 (PO.sub.4).sub.2 O hydrolyzes to hydroxyapatite. Therefore, use of a calcium phosphate dental cement should provide a non-irritating cement capable of setting to a hard consistency and, when desired, remineralizing the dental tissue it contacts. Such a cement would be of great benefit, for example, as a root canal or root surface cement.
Single calcium phosphate cements are incapable of setting to a hard consistency, however, and would suffer from the same drawbacks described above for single calcium phosphate remineralizers. They cannot maintain a relatively constant pH and do not have sufficient remineralization capacity. Through U.S. Pat. No. 3,913,229 (Driskell et al) discloses putty-like pastes containing .alpha.--Ca.sub.3 (PO.sub.4).sub.2, .beta.--Ca.sub.3 (PO.sub.4).sub.2, CaHPO.sub.4 and mixtures thereof as pulp capping, root canal, and tooth replanting materials, it is believed that none of these pastes harden into cements. Furthermore, no remineralization properties are disclosed and it is believed that none of these pastes are capable of any substantial remineralization. Thus, there is a need for a dental cement that is non-irritating, yet has good remineralizing capacity coupled with a stable pH.